Valve has a special Halloween surprise for all the gamers on Steam out there: zombies. Hooray, zombies!

The award winning Left 4 Dead 2 will be free to play, starting today and lasting until Monday, for anyone who uses Steam. So if you haven’t gotten a chance to try out this cooperative zombie survival shooter yet, what in the name of All Hallow’s Eve are you waiting for?

To motivate current players to get their friends into the zombie-killing business, Valve has actually made a brand new achievement that awards current players who get new users to play the game. The achievement, called “Good Guy Nick,” requires you to play a game with free weekend players and help them to survive a campaign. Unfortunately, there is no way of knowing who is a free weekend player, so you’ll either have to talk with your teammates to find out, or orchestrate your own meetup with friends who don’t yet have the game. The fact that there is an achievement based on free weekends of play makes it seem likely that we’ll see more free weekends in the future.